FIG. 1 is a block diagram showing an exemplary configuration of a video display device in conformity with the field sequential color scheme.
The field sequential color scheme refers to a scheme for displaying a color video on a frame-by-frame basis by displaying images corresponding to red (R), green (G), blue (B), or mixed colors comprised of two or more of these colors (for example, yellow (Y), cyan (C), magenta (M), white (W, clear)) on a display panel in an arbitrary order, and sequentially irradiating the display panel with light of colors corresponding to the images to be displayed, from the front or back of the display panel.
As shown in FIG. 1, a video display device of field sequential color scheme comprises video processing circuit 1, scaler circuit 2, LED (light source) driving circuit 3, liquid crystal driving circuit 4, LED (light source) drivers 5-7, LEDs 8-10, panel driver 11, cross dichroic prism 12, liquid crystal panel 13, and projection lens 14.
Video processing circuit 1 performs an A/D conversion of a video signal supplied from the outside, and predetermined video signal processing in conformity with a video standard.
Scaler circuit 2 performs scaling (conversion of resolution through signal interpolation or reduction) for a video signal output from video processing circuit 1 in accordance with the resolution of liquid crystal panel 13 which is used as a display panel.
LED driving circuit 3 generates driving signals for lighting LEDs 8-10, which are light sources of respective colors (red (R), green (G), blue (B)) required to display a color video, which is to be irradiated onto liquid crystal panel 13, in accordance with a video signal output from scaler circuit 2.
LED drivers 5-7 light LEDs 8-10 in red (R), green (G), blue (B) in accordance with the driving signals output from LED driving circuit 3.
Liquid crystal driving circuit 4 generates an image signal for displaying an image corresponding to each color on liquid crystal panel 13 in an arbitrary order in accordance with the video signal output from scaler circuit 2.
Panel driver 11 displays the image corresponding to each color on liquid crystal panel 13 in accordance with the image signal output from liquid crystal driving circuit 4.
Cross dichroic prism 12 irradiates the colors emitted by LEDs 8-10 of red (R), green (G), blue (B), or light of mixed colors comprised of two or more of these colors from the back side of liquid crystal panel 13.
Liquid crystal panel 13, which is, for example, a transmission-type liquid crystal panel, sequentially displays images corresponding to the respective colors in accordance with the image signal output from liquid crystal driving circuit 4.
Projection lens 14 projects a video which has transmitted liquid crystal panel 13 onto screen 15 or the like.
FIGS. 2-1 to 2-6 are timing charts showing exemplary operations of the video display device of field sequential color scheme in the background art. Notably, a method of driving liquid crystal panel 13 and LEDs 8-10 in conformity with the field sequential color scheme, shown in FIGS. 2-1 to 2-6, is also described, for example, in Japanese Laid-Open Patent Application No. 2003-241714A.
FIG. 2-1 is an example where one frame is divided into three sub-frames, images corresponding to respective colors of one frame are displayed on liquid crystal panel 13 in the order of red (R), green (G), and blue (B) in the three sub-frames, and LEDs 8-10 corresponding to the colors on the displayed images are sequentially lit to display a color image.
In the example shown in FIG. 2-1, red (R) image data is written into liquid crystal panel 13 by liquid crystal driving circuit 4 in the first sub-frame of one frame, and after the completion of displaying an image by liquid crystal panel 13, red (R) LED 8 is lit by LED driving circuit 3 at an appropriate luminance in accordance with a gradation level of the displayed image.
Similarly, green (G) image data is written into liquid crystal panel 13 by liquid crystal driving circuit 4 in the second sub-frame of the one frame, and after completion of displaying an image by liquid crystal panel 13, green (G) LED 9 is lit by LED driving circuit 3 at an appropriate luminance in accordance with a gradation level of the displayed image.
Further, blue (B) image data is written into liquid crystal panel 13 by liquid crystal driving circuit 4 in the last sub-frame of the one frame, and after the completion of displaying an image by liquid crystal panel 13, blue (B) LED 10 is lit by LED driving circuit 3 at an appropriate luminance in accordance with a gradation level of the displayed image.
Generally, in liquid crystal panel 13, a certain period of time is taken from the time image data to be displayed is written into TFTs (Thin Film Transistor), not shown, arrayed on a pixel-by-pixel basis to the time that an image is actually displayed on the basis of the image data. A “panel video writing” period shown in (c) of FIGS. 2-1 to 2-6 is a period of time which has been previously set in consideration of a period of time which is taken from the start of writing of image data into TFTs to the completion of displaying the image.
FIG. 2-2 is an example where one frame is divided into four sub-frames, images corresponding to respective colors of one frame are displayed on liquid crystal panel 13 in the order of white (W, clear), red (R), green (G), and blue (B) in the four sub-frames, and LEDs 8-10 corresponding to the colors on the displayed images are sequentially lit to display a color image. The illumination of white (W, clear) is achieved by simultaneously lighting LEDs 8-10 of red (R), green (G), and blue (B) and combining the illumination light with cross dichroic prism 12.
Operations of liquid crystal driving circuit 4 and LED driving circuit 3 in each sub-frame are similar to those in the example shown in FIG. 2-1 except that a different LED should be lit. Accordingly, a description thereon is herein omitted. The same applies to exemplary operations shown later in FIGS. 2-3 to 2-6 as well.
FIG. 2-3 is an example where one frame is divided into four sub-frames, images corresponding to respective colors of one frame are displayed on liquid crystal panel 13 in the order of yellow (Y), blue (B), green (G), and red (R) in the four sub-frames, and LEDs 8-10 corresponding to the colors on the displayed images are sequentially lit to display a color image. The illumination of yellow (Y) is achieved by simultaneously lighting LEDs 8, 9 of red (R) and green (G) and combining the illumination light with cross dichroic prism 12.
FIG. 2-4 is an example where one frame is divided into five sub-frames, images corresponding to respective colors of one frame are displayed on liquid crystal panel 13 in the order of yellow (Y), blue (B), green (G), red (R), and cyan (C) in the five sub-frames, and LEDs 8-10 corresponding to the colors on the displayed images are sequentially lit to display a color image. The illumination of cyan (C) is achieved by simultaneously lighting LEDs 9, 10 of green (G) and blue (B), and combining the illumination light with cross dichroic prism 12.
FIG. 2-5 is an example where one frame is divided into six sub-frames, images corresponding to respective colors of one frame are displayed on liquid crystal panel 13 in the order of yellow (Y), blue (B), magenta (M), green (G), red (R), and cyan (C) in the six sub-frames, and LEDs 8-10 corresponding to the colors on the displayed images are sequentially lit to display a color image. The illumination of magenta (M) is achieved by simultaneously lighting LEDs 8, 10 of red (R) and blue (B), and combining the illumination light with cross dichroic prism 12.
FIG. 2-6 is an example where one frame is divided into six sub-frames, images corresponding to respective colors of one frame are displayed on liquid crystal panel 13 in the order of yellow (Y), blue (B), white (W, clear), green (G), red (R), and cyan (C) in the six sub-frames, and LEDs 8-10 corresponding to the colors on the displayed images are sequentially lit to display a color image.
In the field sequential color scheme, one frame is divided into a plurality of sub-frames, and images of red (R), green (G), blue (B), or mixed colors comprised of two or more of these colors are sequentially displayed to display a color image, taking advantage of the persistence of human's vision. As such, a video display device of field sequential color scheme is known to suffer from flicker, color breakup and the like on a video displayed on a screen or the like.
In order to suppress flicker to such a degree that a person does not feel discomfort, for example, even in a three-plate configuration which comprises three liquid crystal panels for red (R), green (G), and blue (B), it is necessary to set a refresh rate, which is a screen rewriting speed, to 60 Hz or higher at a minimum. For this reason, in a single plate configuration which employs a single liquid crystal panel, as shown in FIG. 1, the refresh rate is required to be 180 Hz or higher, i.e., at least three times higher than the 60 Hz rate.
Conventional video display devices of field sequential color scheme have addressed such problems as flicker, color breakup and the like by increasing the number of sub-frames which make up one frame, and increasing the refresh rate. Specifically, problems such as flicker, color breakup and the like have been addressed by using LEDs or laser light, which can be turned on/off at high speeds, as a light source, or by using a DMD (Digital Micro-mirror Deice), which can be driven at a refresh rate higher than that of a liquid crystal panel, as a display panel.
However, since DMD is generally expensive, a liquid crystal panel is preferably employed for a display panel in order to restrain an increase in the cost of a video display device. However, current liquid crystal panels can respond at a speed of approximately 1.5 ms at most, when an image is displayed in the sequence of white, black, and white, and therefore require one horizontal scanning period of at least approximately 2 μs in consideration of a through rate of a panel driver, and a time taken to write image data into TFTs (Thin Film Transistor) which form part of the liquid crystal panel. Consequently, the number of sub-frames in one frame is limited to approximately six at present.
Also, since the panel driver is restricted in operation frequency to an upper limit value of approximately 150 MHz, the number of sub-frames making up one frame is also limited from this viewpoint.
Accordingly, a novel approach is needed, other than increasing the refresh rate, in order to reduce color breakup to further improve the image quality of videos displayed on a screen or the like.